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| United States Patent |
6,241,937
|
|
Nakamura
, et al.
|
June 5, 2001
|
Process for the production of polyolefin moldings
Abstract
A polyolefin shaped article having high strength and high modulus is
efficiently manufactured without a need to accompany a process of removing
a flow modifier or the like.
A method is characterized as including the steps of melt forming a
composition principally containing 100 parts by weight of high-density
polyolefin, such as high-density polyethylene, and 1-30 parts by weight of
a polymerizable monomer or oligomer having a preferred affinity for the
high-density polyolefin into a sheet or film, calendering and stretching
the sheet or film, and then polymerizing the polymerizable monomer or
oligomer incorporated in the sheet or film.
| Inventors:
|
Nakamura; Masanori (Kyoto, JP);
Yamamoto; Satoru (Kyoto, JP)
|
| Assignee:
|
Sekisui Chemical Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
284950 |
| Filed:
|
April 27, 1999 |
| PCT Filed:
|
October 30, 1997
|
| PCT NO:
|
PCT/JP97/03947
|
| 371 Date:
|
April 27, 1999
|
| 102(e) Date:
|
April 27, 1999
|
| PCT PUB.NO.:
|
WO98/18611 |
| PCT PUB. Date:
|
May 7, 1998 |
Foreign Application Priority Data
| Current U.S. Class: |
264/470; 264/210.2; 264/210.6; 264/210.7; 264/290.2; 264/477 |
| Intern'l Class: |
B29C 071/04; B29C 047/94 |
| Field of Search: |
264/470,477,210.7,210.6,290.2
425/363,210.2
|
References Cited
U.S. Patent Documents
| 5505900 | Apr., 1996 | Suwanda et al.
| |
| Foreign Patent Documents |
| 0206672A | Dec., 1986 | JP.
| |
Primary Examiner: Silbaugh; Jan H.
Assistant Examiner: Eashoo; Mark
Attorney, Agent or Firm: Townsend & Banta
Claims
What is claimed is:
1. A method for manufacture of a polyolefin shaped article comprising the
steps of:
(a) first melt forming a composition principally containing 100 parts by
weight of high density polyolefin and 3-30 parts by weight of a
polymerizable unsaturated compound having a preferred affinity for the
high-density polyolefin into a sheet or film,
(b) calendering said sheet or film,
(c) stretching the calendered sheet or film, and
(d) then polymerizing the polymerizable unsaturated compound having flow
modifier properties incorporated in the stretched sheet or film,
wherein the polyolefin shaped article has a tensile strength of at least
0.40 GPa and a tensile modulus of at least 16.1 GPa, and the polymerized
unsaturated compound remains in the polyolefin shaped article.
2. The method for manufacture of a polyolefin shaped article as recited in
claim 1, wherein said high-density is polyolefin is high-molecular weight
polyolefin.
3. The method for manufacture of a polyolef in shaped article of claim 2,
wherein said composition further contains a free-radical initiator.
4. The method for manufacture of a polyolefin shaped article of claim 2,
wherein said polymerizable unsaturated compound is polymerized by electron
beam irradiation.
5. The method for manufacture of a polyolefin shaped article as recited in
claim 1, wherein said composition further contains a free-radical
initiator.
6. The method of manufacture of a polyolefin shaped article of claim 5,
wherein said free-radical initiator is a photoinitiator and the step of
polymerizing the polymerizable unsaturated compound incorporated in the
stretched sheet or film is carried out by ultraviolet irradiation.
7. The method for manufacture of a polyolefin shaped article of claim 1,
wherein the step of polymerizing said polymerizable unsaturated compound
is carried out by electron beam irradiation.
8. The method for manufacture of a polyolefin shaped article of claim 1,
wherein at least a part of said polymerizable unsaturated compound is a
trifunctional compound.
9. The method for manufacture of a polyolefin shaped article of claim 1,
wherein, in the step of calendering the sheet or film, the calendering is
effected at a calender ratio in the range of 2-10.
10. The method for manufacture of a polyolefin shaped article of claim 1,
wherein, in the step of stretching the sheet or film, the stretching is
effected at a stretch ratio of at least 2.
11. The method for manufacture of a polyolefin shaped article of claim 1,
wherein said high-density polyolefin is high-density polyethylene having a
weight-average molecular weight in the range of 200,000-500,000.
12. The method for manufacture of a polyolefin shaped article in claim 11,
wherein the calendering is effected at a temperature within the range of
70-120.degree. C.
13. The method for manufacture of a polyolefin shaped article of claim 12,
wherein the stretching is effected at a temperature within the range of
from 70.degree. C. to a melting point of said high-density polyethylene.
14. The method for manufacture of a polyolefin shaped article in claim 11,
wherein the stretching is effected at a temperature within the range of
from 70.degree. C. to a melting point of said high-density polyethylene.
15. The method for manufacture of a polyolefin shaped article of claim 1,
wherein further comprising a step of subjecting the sheet or film to heat
treatment, at least prior to the stretching step.
16. The method for manufacture of a polyolefin shaped article of claim 1,
wherein the stretching step comprises multi-stage stretching steps which
effect stretching equidirectionally at two or more stages, and said
multi-stage stretching steps include a final-stage stretching step which
effects equidirectional stretching at an orientation temperature within a
range of 35.degree. C. to 5.degree. C. reduction from an orientation
temperature used in a stretching step immediately before the final-stage
stretching step.
Description
TECHNICAL FIELD
The present invention relates to a method for manufacture of polyolefin
shaped articles having high strength and high modulus.
BACKGROUND ART
For manufacture of polyolefin having high strength and modulus, Japanese
Patent Laying-Open No. Hei. 1-280013 discloses a method wherein ultra-high
molecular weight polyethylene and a flow modifier such as paraffin wax are
melt kneaded at a temperature above a melting point of the ultra-high
molecular weight polyethylene and extruded from a die into a shaped
article which is subsequently subjected to multi-stage stretching. The
publication states that such a method permits the highly productive and
simple manufacture of ultra-high molecular weight polyethylene having high
strength and modulus on an industrial scale.
However, the method described in the above-identified publication requires
the use of a solvent to remove the flow modifier either during or
subsequent to the stretching process. This not only adds to operating time
but also poses a problem of increased manufacturing cost resulting from
the need to use the solvent for removal of the flow modifier.
DISCLOSURE OF THE INVENTION
An object of the present invention resides in its provision of a
manufacturing method by which a polyolefin shaped article having high
strength and modulus can be efficiently manufactured, thus obviating a
need for a process of removing the flow modifier.
The present invention, as recited in claim 1, is a method for manufacture
of a polyolefin shaped article, which is characterized as including the
steps of melt forming a composition principally containing 100 parts by
weight of high-density polyolefin and 1-30 parts by weight of a
polymerizable unsaturated compound having a preferred affinity for the
high-density polyolefin into a sheet or film, calendering the sheet or
film obtained, stretching the calendered sheet or film, and polymerizing
the polymerizable unsaturated compound incorporated in the stretched sheet
or film.
In the method for manufacture of a polyolefin shaped article in accordance
with the present invention, high molecular weight polyolefin is preferably
employed for the above-specified high-density polyolefin, as recited in
claim 2.
Also, in a particular aspect of the present invention, the aforementioned
composition further contains a free-radical generator, as recited in claim
3.
Preferably, a photoinitiator is employed for the above free-radical
generator and the step of polymerizing the polymerizable unsaturated
compound, which follows the stretching step, is carried out by ultraviolet
irradiation, as recited in claim 4.
Alternatively, the step of polymerizing the polymerizable unsaturated
compound may be performed by electron beam irradiation, as recited in
claim 5.
Also, in a particular aspect of the present invention, the aforementioned
polymerizable unsaturated compound consists at least partly of a
trifunctional compound, as recited in claim 6.
In the method for manufacture of a polyolefin shaped article in accordance
with the present invention, in the step of calendering the aforementioned
sheet or film, the calendering is performed at a calender ratio preferably
within the range of 2-10, as recited in claim 7.
It is also preferred that, in the step of stretching the aforementioned
sheet or film, the stretching is performed at a stretch ratio preferably
of at least 2, as recited in claim 8.
In a narrower aspect of the present invention, high-density polyethylene
having a weight average molecular weight within the range of
200,000-500,000 is employed for the aforementioned high-density
polyolefin, as recited in claim 9.
Where high-density polyethylene is employed as the aforementioned
high-density polyolefin, it is preferred that, in the calendering step,
the calendering is performed at a temperature within the range of
70-120.degree. C., as recited in claim 10. It is also preferred that, in
the stretching step, the stretching is carried out at a temperature
ranging from 70.degree. C. to a melting point of high-density
polyethylene, as recited in claim 11.
Preferably, the method for manufacture of a polyolefin shaped article in
accordance with the present invention further includes a step of
subjecting the sheet or film, at least prior to being stretched, to heat
treatment as recited in claim 12.
Also, the above-specified stretching step may be a multi-stage stretching
step wherein stretching is effected in the same direction as a calendering
direction by two or more stages and wherein stretching at a final stage is
effected equi-directionally at an orientation temperature within a range
of 35.degree. C. to 5.degree. C. reduction from an orientation temperature
used in stretching at a stage immediately before the final stage, as
recited in claim 13.
A detailed explanation of the present invention is given below.
The high-density polyolefin resin for use in the present invention is not
particularly specified, so long as it is highly crystalline. Useful
high-density polyolefin resins include homopolymers such as polyethylene,
polypropylene, 1-butene, and 1-pentene; and copolymers containing not
higher than 10 weight % of copolymeric components such as vinyl monomer
units including vinyl acetate, vinyl alcohol, vinyl chloride and acrylic
acid.
Preferably, high-molecular weight high-density polyolefin having a high
weight-average molecular weight is suitably used for the high-density
polyolefin resin, since it is able to impart increased mechanical
properties, such as strength and modulus, to a resulting polyolefin shaped
article. The high-molecular weight polyolefin, as used herein, refers to
polyolefin which, like the below-described high-density high-molecular
weight polyethylene, has a weight-average molecular weight of a level
sufficiently high to impart increased mechanical properties, such as
strength and modulus. Its weight-average molecular weight depends upon the
particular type of polyolefin used, but is generally not lower than
200,000.
In the present invention, preferred among the above-listed resins is the
high-density polyethylene resin.
A useful high-density polyethylene resin has a weight-average molecular
weight preferably in the range of 200,000-500,000. If the weight-average
molecular weight is below 200,000, stretching to orientate molecular
chains may result in the failure to achieve desired improvements in
strength and modulus. On the other hand, if the weight-average molecular
weight exceeds 500,000, melt formation may fail, as is typically the case
with ultra-high-molecular-weight polyethylene. Accordingly, the use of
high-density high-molecular weight polyethylene having a weight-average
molecular weight in the range of 200,000-500,000, as specified above,
results in the further increased mechanical properties, such as strength
and modulus, relative to using low molecular weight polyethylene.
Also, where a high-density polyethylene resin is used, its density is not
lower than 0.94 g/cm.sup.3. If the density is low, stretching may result
in the failure to achieve the desired improvements in strength and
modulus. Also, its melt index (MI) is preferably in the range of 0.1-20,
more preferably in the range of 0.1-10. If the melt index is lower than
0.1, an increased load may be imposed on a fabricating machine such as an
extruder. If it exceeds 20, fabrication may become difficult.
In the present invention, a composition which contains 100 parts by weight
of high-density polyolefin and 1-30 parts by weight of polymerizable
unsaturated compound having a preferred affinity for the high-density
polyolefin is used as a starting material. If the polymerizable
unsaturated compound content is less than 1 part by weight, the
incorporation thereof may not always be effective to achieve expected
improvement in stretchability. On the other hand, if it exceeds 30 parts
by weight, the polymerizable unsaturated compound may be caused to bleed
on a surface of a shaped article, which not only lowers handling
characteristics but also adversely affects stretchability.
The present invention will be now described in more detail with reference
to an exemplary case where a high-density polyethylene resin is used as a
representative high-density polyolefin. It should be recognized, however,
that the use of the other types of polyolefins also results in the
successful manufacture of polyolefin shaped articles, if properly
processed in a manner as described below.
The aforementioned polymerizable unsaturated compound having a preferred
affinity for a high-density polyethylene resin is not particularly
specified, so long as it is a low molecular weight unsaturated compound
having a basic nature of being readily mixed in the high-density
polyethylene resin. Various types of polymerizable monomers and/or
polymerizable oligomers can be employed for the polymerizable unsaturated
compound. Examples of those polymerizable monomers and oligomers include
acrylic monomers and oligomers, and vinyl monomers and oligomers.
Specific examples of polymerizable monomers include trimethylolpropane
triacrylate, trimethylolpropane trimethacrylate, diallyl phthalate,
triallyl isocyanurate, hexanediol diacrylate, dipentaerythritol
pentacrylate, isobornyl acrylate, and 2-phenoxyether acrylate.
Examples of polymerizable oligomers include epoxy acrylate, urethane
acrylate, polyester acrylate, silicone acrylate, as well as special
oligomers such as liquid butadiene.
Particularly suitable for use as the aforementioned polymerizable
unsaturated compounds are trifunctional compounds, such as
trimethylolpropane triacrylate, trimethylolpropane trimethacrylate and
triallyl isocyanurate, since they are highly reactive and the use thereof
reduces a tendency that the polymerizates, subsequent to being stretched,
exudate onto their surfaces.
In the present invention, a free-radical generator, as illustrated by
various types of organic peroxides and photoinitiators, may preferably be
added to promote polymerization of the aforementioned polymerizable
monomers and oligomers. The free-radical generator may be added in the
amount preferably of 3-5 parts by weight, based on 100 parts by weight of
the polymerizable unsaturated compound. Specific examples of the
aforementioned organic peroxides include di-tert-butyl peroxide, dicumyl
peroxide, tert-butyl cumyl peroxide and the like. Specific examples of
photoinitiators include acetophenone, benzophenone, thioxanthone, benzoin,
Michler's ketone and the like.
In a case where the free-radical generator is a photoinitiator,
incorporation thereof in a sheet or film, i.e., in a shaped article, can
be achieved by a method wherein a composition prepared, prior to being
fabricated, by mixing a high-density polyethylene resin, a polymerizable
unsaturated compound and a photoinitiator is melt kneaded in an extruder
during fabrication, for example. In the case of organic peroxide, it may
be pregnated in a shaped article by a method wherein the shaped article
containing a melt kneaded mixture of high-density polyethylene and a
polymerizable unsaturated compound is immersed in a solvent into which
organic peroxide has been previously dissolved.
A photoinitiator is a particularly preferred free-radical generator for its
nature to be readily mixed and easily handled. Among photoinitiators,
benzophenone and thioxanthone are particularly suitable for use as the
free-radical generator.
When the aforementioned composition is melt processed as by an extruder, a
melt temperature is chosen to be preferably not lower than 130.degree. C.,
more preferably not lower than 140.degree. C. If the melt temperature
falls below 130.degree. C., incomplete melting of the resin likely results
to reduce its compatibility with the polymerizable unsaturated compound,
possibly leading to reduced stretchability in the subsequent stretching
process. On the other hand, if the melt temperature is increased
excessively, the high-density polyethylene resin and photoinitiator may be
caused to decompose or evaporate. Thus, the melt temperature is preferably
controlled not to substantially exceed 250.degree. C.
In the present invention, the use of high-density high-molecular weight
polyethylene imparts further increased mechanical properties, such as
strength and modulus, relative to using low molecular weight
polyethylenes. Where high-molecular weight polyethylene is. used, it is
preferred that its weight-average molecular weight is at least
2.times.10.sup.5. If the weight-average molecular weight is below this
value, stretching to orientate molecular chains unlikely improves strength
and modulus. On the other hand, if a polyethylene molecular weight is
increased excessively, melt fabrication thereof may become impossible, as
is typically the case with ultra-high-molecular-weight polyethylene. It is
thus preferred that its weight-average molecular weight does not
substantially exceed 5.times.10.sup.5.
In the present invention, the melt composition prepared in the manner as
stated above is first formed into a sheet or film. Any forming technique
can be employed including conventional rolling and calendering techniques.
An initial thickness of the shaped article exerts a significant influence
on the subsequent calendering process. That is, as the thickness of the
shaped article increases, a force required for calender rolls in
compressing the shaped article, i.e., a so-called pressure force, becomes
greater. This causes increased deflection of calender rolls to result in
the failure to achieve uniform calendering in a width direction. On the
other hand, if the shaped article is excessively thin, the shaped article
after being subjected to calendering is also thinned excessively. In
addition, opposing calender rolls are caused to contact with each other to
possibly result in the reduction of service lives thereof. Accordingly,
the thickness of the shaped article is preferably in the range of 0.5-4
mm.
In the present invention, the sheet or film formed in the manner as
described above may be subjected to heat treatment, prior to being
calendered or stretched. Subjecting the sheet or film to heat treatment is
preferred for its effectiveness to arrange crystalline state thereof,
facilitate subsequently-performed stretching operation and improve a
degree of orientation.
The aforementioned heat treatment is not particularly limited, but is
preferably performed under pressure in a closed container to prevent the
polymerizable unsaturated compound melt-mixed in the sheet from escaping
in the form of vapor.
In the present invention, the sheet or film formed in the manner as
described above is then rolled or calendered. Calendering may be carried
out by using a commonly-employed technique, i.e., by providing a
clearance, smaller in dimension than the thickness of the shaped article,
between a pair of counterrotating calender rolls, and introducing the
shaped article between the calender rolls so that the shaped article is
reduced in thickness and extended. If a temperature of the calender rolls
in a calendering process is excessively low, uniform calendering may be
impaired by the increased pressure force. On the other hand, if it is
excessively high, the shaped article may be melt broken. Therefore, in the
calendering process, the temperature of the calender rolls is kept
preferably within the range of 70-120.degree. C., more preferably within
the range of 90-115.degree. C. Also, an excessive reduction in calender
ratio may result not only in the failure to gain an expected calendering
effect, but also in imposing the increased burden on the subsequent
stretching process. On the other hand, if the calender ratio is
excessively high, uniform calendering may be impaired by the increased
pressure force. In addition, the shaped article may be thinned excessively
to result in breakage thereof when subsequently subjected to stretching.
Accordingly, the calender ratio is preferably in the range of 2-10. The
calender ratio, as used herein, is defined by the following relationship:
(Calender ratio)=(thickness of the shaped article prior to being
calendered)/(thickness of the shaped article after being calendered)
In the present invention, the sheet or film calendered in the manner as
stated above is then stretched. A variety of stretching techniques can be
employed including conventional roll and zone stretching techniques. The
roll stretching technique refers to a technique wherein a pair of rolls
rotating at different speeds is operated to interpose a sheet therebetween
while heating the sheet so that the sheet is stretched, and is a technique
which can orientate molecular chains predominantly uniaxially. In this
case, a speed ratio of the pair of rolls gives a stretch ratio. The zone
stretching technique is a technique which locally stretches a sheet by
heating local areas of the sheet by a heater or a hot air, and accordingly
presents such benefits that the sheet while stretched is restrained from
thermally degrading and that the reduction in width of the sheet is
minimized to permit uniform stretching of the sheet.
In the present invention, the roll stretching technique is suitably
employed which requires no special equipment, which can readily control a
stretch ratio and which is highly productive. Techniques to heat the
shaped article include hot air, hot water, infrared and microwave heating.
Among these, hot air heating is suitably employed which can be performed
by using simple equipments and is able to control a temperature without
difficulty. The stretching may be carried out in a single or multiple
stages. Generally, a multi-stage stretching technique, which effects
repeated stretching at low stretch ratios, reduces the occurrence of
breakage of a sheet while stretched to insure stable stretching thereof.
However, the increased number of stage, while effective in obtaining
enhanced stability and increased stretch ratio, accompanies scale-up of
equipments to be used. Accordingly, stretching may preferably be performed
substantially in two to five stages.
Also, an orientation temperature is chosen to fall within the range from
70.degree. C. to a melting temperature of high-density polyethylene used,
more preferably within the range from 80.degree. C. to a temperature
10.degree. C. lower than the melting point of polyethylene. The melting
point, as used herein, refers to a maximum point of endothermic peaks
accompanying fusion of crystal, which can be identified in the thermal
analysis as by a differential scanning calorimeter (DISC). If the
stretching is effected at a temperature lower than 70.degree. C.,
whitening of a shaped article may result. If effected at a temperature
above the melting point, stretching may not always be effective to achieve
desired improvement in strength to possibly result in breakage of the
shaped article.
In the stretching process, the stretch ratio is chosen to be preferably not
less than 2, more preferably in the range of 3-10. If the stretch ratio is
below 2, desired improvements in strength and modulus of the shaped
article may not be obtained. On the other hand, care should be taken when
increasing the stretch ratio. The excessively increased stretch ratio may
result in the increased occurrence of breakage of the shaped article
while. stretched. The stretch ratio, as used herein, is defined by the
following relationship:
(Stretch ratio)=(sectional area of the shaped article prior to being
stretched)/(sectional area of the shaped article after being stretched).
As stated earlier, the orientation temperature is desired to fall within
the range from 70.degree. C. to the melting point of high-density
polyethylene used. Particularly in the case of the aforementioned
multi-stage stretching wherein a sheet is stretched in the same direction
as a calendering direction by two or more stages, the below-stated result
may favorably arise if stretching at a final stage is effected at an
orientation temperature within a range of 35.degree. C. to 5.degree. C.
reduction from an orientation temperature used in stretching the sheet at
a stage immediately before the final stage. That is, if the orientation
temperature at the final stage is reduced to a level as specified above,
microvoids can be forcibly and stably produced in the sheet to possibly
result in the density reduction of the sheet, which is favored in some
cases.
In such a case, if the orientation temperature at the final stage goes
beyond a temperature reduced by 5.degree. C. from the orientation
temperature used in stretching the sheet at the stage immediately before
the final stage, the multi-stage stretching may result in the failure to
stably produce microvoids, accordingly leading to the failure to achieve
the adequate density reduction of the sheet. In addition, non-uniform
production of voids may result in the reduced uniformity of the sheet. On
the other hand, if the orientation temperature at the final stage falls
below a temperature reduced by 35.degree. C. from the orientation
temperature used in stretching at the stage immediately before the final
stage, the excessively reduced temperature may result in the failure to
achieve smooth stretching at the final stage.
The sheet in which the aforementioned microvoids are produced is usually in
a stable whitened state. On the other hand, unless such production of
microvoids is desired, the sheet may be further stretched immediately
after the final stage to remove those microvoids.
In the present invention, the polymerizable unsaturated compound
incorporated in the sheet or film after being stretched is not removed but
subjected to polymerization. By subjecting to polymerization, an extremely
time-consuming procedure to remove such compound can be omitted, leading
to a marked improvement in productivity. Polymerization of the
polymerizable unsaturated compound can be achieved by heating or by
irradiation with ultraviolet or electron rays. The ultraviolet irradiation
is most preferred. If heating is selected, the applied heat may cause a
shaped article to shrink, possibly resulting in the reduction of physical
properties thereof once improved by the stretching. At worst, the shaped
article may be caused to melt. If the electron beam irradiation is
selected, the requirement of expensive equipments raises an initial cost,
leading to the increase in price of the shaped article. When the
ultraviolet irradiation is selected, the aforementioned photoinitiator may
preferably be incorporated in the shaped article to promote
polymerization.
(FUNCTION)
In the present invention, mechanical properties of a shaped article,
typically in tensile strength and modulus, can be improved by stretching
the shaped article which have been previously improved in extensibility by
inclusion of the polymerizable unsaturated compound, such as a polymeric
monomer or polymeric oligomer, which is an alternative to a plasticizer.
Also, the polymerizable unsaturated compound is subjected to
polymerization after the stretching process, so that a
conventionally-employed procedure for removal thereof is made unnecessary.
This accordingly permits the highly productive manufacture of polyolefin
shaped articles having high strength and modulus. Besides, since the
polymerizable unsaturated compound is subjected to polymerization after
the stretching process, its migration onto a shaped article surface,
so-called bleeding phenomenon, can be prevented.
In accordance with the present invention, high-density polyolefins,
particularly high-density high-molecular weight polyolefins having
weight-average molecular weights in the approximate range of
200,000-500,000 can be subjected to stretching to result in a marked
improvement in strength and modulus thereof, while conventionally
considered to be very difficult due to a large degree of entanglement of
molecular chains.
Also in the present invention, a sheet or film may be subjected to heat
treatment prior to being calendered or stretched. Subjecting the sheet or
film to heat treatment for at least an hour at a temperature sufficient to
cause adequate transformation of crystals, which in the case of a
polyethylene resin is within the range from 70.degree. C. to a melting
point of the polyethylene resin, is effective to impart an increased
degree of crystallinity. This is considered to result in a reduced degree
of entanglement between polyethylene molecules. Such a heat treatment
assures a smooth performance of the subsequent stretching, leading
accordingly to the successful improvement in degree of orientation.
EMBODIMENTS OF THE INVENTION
Examples 1-17 and Comparative Examples 1-8
(1) Preparation of Substrate Sheets or Films
Various types of high-density polyolefins, as listed in Tables 1 and 2,
were used. More specifically, high-density polyethylene having a
weight-average molecular weight of 3.3.times.10.sup.5 and a melting point
of 135.degree. C. (manufactured by Mitsubishi Chem. Co., Ltd., grade:
HY540, density: 0.961) was used in Examples 1-3, 6-9 and 11-17 and in
Comparative Examples 1-4 and 6; high-density polyethylene having a
weight-average molecular weight of 1.5.times.10.sup.5 and a melting point
of 136.degree. C. (manufactured by Mitsubishi Chem. Co., Ltd., grade:
HJ560W, density: 0.964) in Example 4 and Comparative Example 5;
high-density polyethylene having a weight-average molecular weight of
2.2.times.10.sup.5 and a melting point of 132.degree. C. (manufactured by
Mitsui Petrochem. Co., Ltd., grade: 5000SR, density: 0.958) in Example 5;
and high-density polypropylene having a weight-average molecular weight of
1.5.times.10.sup.5 and a melting point of 165.degree. C. (manufactured by
Mitsubishi Chem. Co., Ltd., grade: MA3) in Example 10 and Comparative
Examples 7 and 8.
Added to any of the aforementioned high-density polyolefins were
trimethylolpropane triacrylate, trimethylolpropane trimethacrylate,
triallyl isocyanurate or divinyl benzene, and benzophenone as a
free-radical initiator, respectively in the amounts as listed in Table 1.
Each of the resulting resin mixtures was melt at a resin temperature of
about 180.degree. C. by using a corotating twin-screw extruder
(manufactured by Ikegai Tekko Co., Ltd., PCM30), and then passed through a
calendering machine maintained at a roll temperature of 80.degree. C., so
that the resin mixture was shaped into a tape form 60 mm wide and 0.7 mm
thick. The amounts given in Table 1 are all expressed in parts by weight,
based on 100 parts by weight of high-density polyolefin used.
As apparent from Table 1, instead of trimethylolpropane trimethacrylate
which is a trifunctional compound, divinyl benzene, a bifunctional
compound, was used in Example 6.
In Example 7, divinyl benzene was not added. This is because polymerization
in Example 7 was carried out by electron irradiation, as described below.
Neither of the aforementioned polymerizable unsaturated compound and
benzophenone was added in Comparative Examples 1, 5 and 7. Benzophenone
was not added in Comparative Example 6.
(2) Heat Treatment Process
Solely in Example 8, a resulting sheet was, prior to being calendered,
subjected to heat treatment. That is, a high-density polyethylene prepared
in the same manner as in Example 2 was heat treated in a container
maintained at 130.degree. C. for 8 hours, prior to being calendered.
(3) Calendering Process
With the exception of Example 15 (a calender ratio of 8.9), the respective
sheets or films were calendered at calender ratios of about 7, as
indicated in Table 1, by using heat rolls (manufactured by Kodaira
Seisakusho Co., Ltd., a roll diameter of 6 inches) controlled at a
temperature of 115.degree. C.
Also, the calendering temperature was controlled at 145.degree. C. in
Examples 10 and Comparative Examples 7 and 8, and at 85.degree. C. in
Example 17.
(4) Stretching Process
A hot-air heating type roll stretcher was employed to stretch the
calendered sheets or films first at a stretch ratio of 1.8 and then at a
maximum stretch ratio that they can be stretched without breakage. During
the stretching, an orientation temperature was maintained at 95.degree. C.
An overall orientation ratio, as given in the following Tables 3 and 4, is
a product of the calender ratio and stretch ratio.
In Example 9, the two-stage stretching was effected at different
temperatures and at different stretch ratios, i.e., the sheet or film
calendered according to the procedure of Example 2 was stretched by the
hot-air heating type roll stretcher, first at a stretch ratio of 1.8 at a
temperature of 95.degree. C. and then at a stretch ratio of 2.7 at an
orientation temperature of 80.degree. C.
In Example 10, both first-stage and second-stage orientation temperatures
were set at 160.degree. C.
(5) Polymerizing Process
The shaped articles, after stretched, were passed at a speed of 10 m/min
through an apparatus equipped with two opposing sets of three
electrodeless ultraviolet lamps (manufactured by Fusion Japan Co., Ltd.),
so that a top surface of each article was exposed to one set of the UV
lamps while a bottom surface thereof was exposed to another set of the UV
lamps. This procedure induced polymerization of polymerizable monomers
incorporated in the shaped articles.
As can be seen from Table 2, the UV irradiation was not carried out in
Comparative Examples 3, 4 and 8.
In Example 7, instead of UV irradiation, EB irradiation was carried out at
a dosage of 10 Mrads for 2 seconds at an ordinary temperature by using an
area-beam type electron beam emitter (manufactured by Nisshin High Voltage
Co., Ltd., product name: Curetron).
The shaped articles thus obtained were measured for tensile strength and
tensile modulus. A testing method is described below. The results are
given in Tables 3 and 4.
TENSILE STRENGTH AND TENSILE MODULUS
A tensile testing machine (manufactured by Orientic Co., Ltd., machine
designation: Tensilon) was employed to conduct measurements according to
JIS K 7113. Dumbbell specimens of size #2 were used.
A surface condition of each specimen was observed. In Tables 3 and 4, the
specimens which showed bleeding at surfaces thereof are indicated by the
expression "occurred" while the specimens which showed no bleeding are
indicated by the expression "not occurred".
TABLE 1
CAL. STR.
RADICAL
Example PE HEAT CAL. TEMP. STR. TEMP.
PUC.sup.2) GENERATOR
No. M.W..sup.1) TRMT. RATIO (.degree. C.) RATIO (.degree.
C.) (PARTS) (PARTS) IRRADIATION
1 330000 UNDONE 7.0 115 3.6 95.
TMPTM.sup.3) : 5 BP.sup.6) : 1 UV
2 330000 UNDONE 7.1 115 4.6 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
3 330000 UNDONE 7.1 115 4.0 95
TMPTM.sup.3) : 25 BP.sup.6) : 1 UV
4 150000 UNDONE 7.0 115 4.3 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
5 220000 UNDONE 7.0 115 4.1 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
6 330000 UNDONE 6.9 115 4.4 95
DVB.sup.4) : 15 BP.sup.6) : 1 UV
7 330000 UNDONE 7.0 115 4.4 95
TMPTM.sup.3) : 15 -- EB
8 330000 DONE 7.0 115 4.9 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
9 330000 UNDONE 7.0 115 4.9 1ST TEMP.: 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
2ND TEMP.: 80
10 PP M.W. UNDONE 6.9 145 4.5 160
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
150000
11 330000 UNDONE 7.1 115 4.5 95
TAI.sup.5) : 15 BP.sup.6) : 1 UV
12 330000 UNDONE 7.3 115 4.0 95
TAI.sup.5) : 3 BP.sup.6) : 1 UV
13 330000 UNDONE 7.0 115 4.4 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 UV
14 330000 UNDONE 7.3 115 4.0 95
TMPTM.sup.3) : 3 BP.sup.6) : 1 UV
15 330000 UNDONE 8.9 115 3.6 95
TAI.sup.5) : 15 BP.sup.6) : 1 UV
16 330000 UNDONE 7.2 115 4.3 110
TAI.sup.5) : 15 BP.sup.6) : 1 UV
17 330000 UNDONE 7.3 85 4.1 95
TAI.sup.5) : 15 BP.sup.6) : 1 UV
.sup.1) M.W. = MOLECULAR WEIGHT
.sup.2) PUC = POLYMERIZABLE UNSATURATED COMPOUND
.sup.3) TMPTM = TRIMETHYLOLPROPANE TRIMETHACRYLATE
.sup.4) DVB = DIVINYL BENZENE
.sup.5) TAI = TRIALLYL ISOCYANURATE
.sup.6) BP = BENZOPHENONE
TABLE 2
COMP. CAL. STR.
EXAM- PE HEAT CAL. TEMP. STR. TEMP.
PUC.sup.2) GENERATOR
PLE No. M.W..sup.1) TRMT. RATIO (.degree. C.) RATIO (.degree.
C.) (PARTS) (PARTS) IRRADIATION
1 330000 UNDONE 7.0 115 3.2 95
-- -- UV
2 330000 UNDONE 6.9 115 3.4 95
TMPTM.sup.3) : 35 BP.sup.6) : 1 UV
3 330000 UNDONE 7.1 115 4.6 95
TMPTM.sup.3) : 15 BP.sup.6) : 1 NONE
4 330000 UNDONE 7.1 115 4.0 95
TMPTM.sup.3) : 25 BP.sup.6) : 1 NONE
5 150000 UNDONE 7.1 115 4.0 95
-- -- UV
6 330000 UNDONE 7.1 115 4.6 95
TMPTM.sup.3) : 15 -- UV
7 PP M.W. UNDONE 7.0 145 3.5 160
-- -- UV
150000
8 PP M.W. UNDONE 6.9 145 4.5 160
TMPTM.sup.3) : 15 BP.sup.6) : 1 NONE
150000
.sup.1) M.W. = MOLECULAR WEIGHT
.sup.2) PUC = POLYMERIZABLE UNSATURATED COMPOUND
.sup.3) TMPTM = TRIMETHYLOLPROPANE TRIMETHACRYLATE
.sup.4) DVB = DIVINYL BENZENE
.sup.5) TAI = TRIALLYL ISOCYANURATE
.sup.6) BP = BENZOPHENONE
TABLE 3
OVERALL BLEEDING
Exam- ORIENTA- THROUGH
ple TION STRENGTH MODULUS AN ORIENTED
No. RATIO*) (GPa) (GPa) SHEET
1 25.2 0.68 38.4 NOT OCCURRED
2 32.7 0.71 43.5 NOT OCCURRED
3 28.4 0.68 39.8 NOT OCCURRED
4 30.1 0.65 36.5 NOT OCCURRED
5 28.7 0.69 38.1 NOT OCCURRED
6 30.4 0.70 40.1 SLIGHTLY
OCCURRED
7 30.8 0.72 42.1 NOT OCCURRED
8 34.3 0.82 46.2 NOT OCCURRED
9 34.3 0.81 45.0 NOT OCCURRED
10 31.1 0.40 16.1 NOT OCCURRED
11 32.0 0.77 44.5 NOT OCCURRED
12 29.2 0.71 40.1 NOT OCCURRED
13 30.8 0.75 42.8 NOT OCCURRED
14 29.2 0.68 39.8 NOT OCCURRED
15 32.0 0.78 45.0 NOT OCCURRED
16 31.0 0.73 43.0 NOT OCCURRED
17 29.9 0.68 37.8 NOT OCCURRED
*)OVERALL ORIENTATION RATIO = PRODUCT OF A CALENDER RATIO AND A STRETCH
RATIO
TABLE 4
COMP. OVERALL BLEEDING
EXAM- ORIENTA- THROUGH
PLE TION STRENGTH MODULUS AN ORIENTED
NO. RATIO*) (GPa) (GPa) SHEET
1 22.4 0.64 33.5 NOT OCCURRED
2 23.5 0.63 32.4 NOT OCCURRED
3 32.7 0.54 30.1 OCCURRED
4 28.4 0.49 28.7 OCCURRED
5 28.4 0.58 31.9 NOT OCCURRED
6 32.7 0.61 33.2 OCCURRED
7 24.5 0.31 10.5 NOT OCCURRED
8 31.1 0.38 14.2 OCCURRED
*)OVERALL ORIENTATION RATIO = PRODUCT OF A CALENDER RATIO AND A STRETCH
RATIO
Examples 1-3 and Comparative Examples 1 and 2 were contemplated to examine
the influence from the amount of trimethylolpropane trimethacrylate. As
can be clearly seen from Tables 1 through 4, the shaped articles of all
Examples exhibit increased tensile strengths and tensile moduli compared
to the shaped article of Comparative Example 1 which excluded
trimethylolpropane trimethacrylate. As can be appreciated from the
comparison between Example 3 and Comparative Example 2, there apparently
exists a maximum effective amount of trimethylolpropane trimethacrylate.
Incorporation thereof in the amount beyond that maximum level is found to
adversely affect mechanical properties of resulting shaped articles.
The shaped articles of Comparative Examples 3 and 4 were not subjected to
polymerization by UV irradiation, while otherwise equivalent to those of
Examples 2 and 3. The results demonstrate that the shaped articles, unless
polymerized, accompany the occurrence of bleeding and the reduction in
mechanical properties, as indicated in Table 4.
As apparent from the comparisons among Examples 4-6, the use of
polyethylene having an increased molecular weight results in the increased
strength and modulus of a resulting shaped article.
As also apparent from the results of Example 6, the use of a bifunctional
compound, divinyl benzene, as a polymerizable unsaturated compound,
results in the formation of the shaped article which exhibits comparable
strength and modulus but a slightly increased degree of bleeding, due to
the reduced reactivity of divinyl benzene, relative to the shaped article
of Example 2 which instead incorporated a trifunctional compound,
trimethylolpropane triacrylate. Yet, the high-density polyethylene sheet
obtained in Example 6 apparently exhibits sufficient strength and modulus
as contrasted with the high-density polyethylene sheets obtained in
Comparative Examples 1-4.
In Example 7, trimethylolpropane trimethacrylate was polymerized without
the aid of benzophenone, i.e., by EB irradiation. Also in this case, the
resulting high-density polyethylene sheet is found to exhibit sufficient
strength and modulus without appreciable occurrence of bleeding at its
surface.
Example 8 followed the procedure of Example 2, except that the
aforementioned heat treatment was carried out prior to the calendering
process. Results demonstrate that the heat treatment contributes to the
enhanced strength and modulus of the high-density polyethylene sheet
obtained in Example 8, compared to those of the high-density polyethylene
sheet obtained in Example 2.
In Example 9, the practice of multi-stage stretching with the reduced
orientation temperature at the final stage thereof is considered to result
in the formation of the high-density polyethylene sheet which showed the
increased overall orientation ratio of 34.1, production of a number of
microvoids and occurrence of whitening. Also, the high-density
polyethylene sheet obtained in Example 9 exhibited a density of 0.85,
while the high-density polyethylene sheet obtained in Example 2 exhibited
a density of 0.95. This demonstrates that the density reduction is
attainable.
In Example 10, the aforementioned high-density polyethylene having a
weight-average molecular weight of 1.5.times.10.sup.5 and a melting point
of 165.degree. C. was used, a calendering temperature and an orientation
temperature set at 145.degree. C. and 160.degree. C., respectively. The
practice of Example 10 also resulted in the formation of the high-density
polypropylene sheet which showed sufficient levels of strength and modulus
without occurrence of bleeding, as apparent from comparison of their
values indicated in Table 3 to physical values, listed in Table 4, of the
sheets obtained in Comparative Examples 7 and 8 which used the same
high-density polypropylene as in Example 10.
Comparative Example 6 followed the procedure of Example 2, except that
benzophenone was not added. Due to the absence of a free-radical
generating source, UV irradiation was not effective to polymerize
trimethylolpropane trimethacrylate, resulting in the appreciable
occurrence of exudation in the oriented sheet.
EFFECTS OF THE INVENTION
As described above, in accordance with the present invention as recited in
claim 1, a polyolefin shaped article having high strength and high modulus
can be efficiently produced without a need to accompany a step of removing
a flow modifier and the like.
Particularly in accordance with the invention as recited in claim 2, the
use of a high-density polyethylene for the high-density polyolefin results
in the efficient manufacture of polyolefin shaped articles having high
strength and high modulus.
In accordance with the invention as recited in claim 3, the further
incorporation of a free-radical initiator in the aforementioned
composition becomes effective when its activation promotes polymerization
of a polymerizable unsaturated compound. This enables efficient production
of polyolefin shaped articles having high strength and high modulus
without a need to include a step of removing the polymerizable unsaturated
compound.
In accordance with the invention as recited in claim 4, a photoinitiator is
used as the free-radical initiator and the step of polymerizing the
polymerizable unsaturated compound, which follows the stretching step, is
carried out by UV irradiation. This unlikely causes the reduction in
mechanical properties of a shaped article resulting from heat shrinkage or
stretching thereof. Also, the use of UV irradiation to activate the
photoinitiator insures ready and complete polymerization of the
polymerizable unsaturated compound with the use of inexpensive facilities.
In the invention as recited in claim 5, the use of EB irradiation to
polymerize the aforementioned polymerizable unsaturated compound unlikely
causes the reduction in mechanical properties of a shaped article
resulting from heat shrinkage or stretching thereof, while insuring
production of polyolefin shaped articles having improved mechanical
properties such as strength and modulus.
In the invention as recited in claim 6, the use of a highly reactive,
trifunctional compound for the above polymerizable unsaturated compound
reduces a possibility for a polymerizate of the polymerizable unsaturated
compound to bleed on a surface of a resulting polyolefin shaped article.
In the invention as recited in claim 7, the sheet or film is calendered at
a calender ratio in the range of 2-10. This results not only in the
reduced burden on the subsequent stretching step, but also in achievement
of uniform calendering which prevents the calendered sheet or film from
being broken in the subsequent stretching step.
In the invention as recited in claim 8, the sheet or film is stretched at a
stretch ratio of at least 2. This results not only in obtaining polyolefin
shaped articles excellent in strength and modulus but also in the reduced
occurrence of breakage of the shaped articles while stretched.
In the invention as recited in claim 9, the use of high-density
high-molecular weight polyethylene having a weight-average molecular
weight in the range of 200,000-500,000, as the high-density polyolefin
resin, results in obtaining polyolefin shaped articles having further
enhanced strength and modulus. In the invention as recited in claim 10,
the practice of calendering at a temperature in the range of
70-120.degree. C. produces an adequate pressure force to result not only
in the achievement of uniform calendering but also in the reduced
occurrence of breakage of polyolefin shaped articles while being
calendered.
In the invention as recited in claim 11, the practice of stretching at a
temperature within the range from 70.degree. C. to a melting temperature
of high-density polyethylene reduces the occurrences of whitening in the
high-density polyethylene shaped articles and of breakage thereof while
stretched, resulting in stably obtaining the strength enhancement effect
expected from the stretching.
In the invention as recited in claim 12, the sheet or film is subjected to
heat treatment, at least prior to being stretched. This results in the
improvement in crystalline arrangement of the sheet or film, which
promotes the orientation enhancement effect of the subsequently-performed
stretching to allow a smoother operation of the stretching step.
In the invention as recited in claim 13, the multi-stage stretching is
performed such that stretching at a final stage is equidirectionally
effected at an orientation temperature within a range of 35.degree. C. to
5.degree. C. reduction from an orientation temperature used in stretching
at a stage immediately before the final stage. This results in the stable
production of microvoids in the polyolefin shaped articles while stretched
at the final stage, leading to the reduced density thereof.
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